The caller should specify the number of items in the
fds
array in
nfds.

The field
fd
contains a file descriptor for an open file.

The field
events
is an input parameter, a bit mask specifying the events the application
is interested in.

The field
revents
is an output parameter, filled by the kernel with the events that
actually occurred.
The bits returned in
revents
can include any of those specified in
events,
or one of the values
POLLERR,
POLLHUP,
or
POLLNVAL.
(These three bits are meaningless in the
events
field, and will be set in the
revents
field whenever the corresponding condition is true.)

If none of the events requested (and no error) has occurred for any
of the file descriptors, then
poll()
blocks until one of the events occurs.

The
timeout
argument specifies an upper limit on the time for which
poll()
will block, in milliseconds.
Specifying a negative value in
timeout
means an infinite timeout.

The bits that may be set/returned in
events
and
revents
are defined in <poll.h>:

POLLIN

There is data to read.

POLLPRI

There is urgent data to read (e.g., out-of-band data on TCP socket;
pseudo-terminal master in packet mode has seen state change in slave).

POLLOUT

Writing now will not block.

POLLRDHUP (since Linux 2.6.17)

Stream socket peer closed connection,
or shut down writing half of connection.
The
_GNU_SOURCE
feature test macro must be defined
(before including
any
header files)
in order to obtain this definition.

POLLERR

Error condition (output only).

POLLHUP

Hang up (output only).

POLLNVAL

Invalid request:
fd
not open (output only).

When compiling with
_XOPEN_SOURCE
defined, one also has the following,
which convey no further information beyond the bits listed above:

POLLRDNORM

Equivalent to
POLLIN.

POLLRDBAND

Priority band data can be read (generally unused on Linux).

POLLWRNORM

Equivalent to
POLLOUT.

POLLWRBAND

Priority data may be written.

Linux also knows about, but does not use
POLLMSG.

ppoll()

The relationship between
poll()
and
ppoll()
is analogous to the relationship between
select(2)
and
pselect(2):
like
pselect(2),
ppoll()
allows an application to safely wait until either a file descriptor
becomes ready or until a signal is caught.

Other than the difference in the precision of the
timeout argument, the following
ppoll()
call:

If
timeout_ts
is specified as NULL, then
ppoll()
can block indefinitely.

RETURN VALUE

On success, a positive number is returned; this is
the number of structures which have nonzero
revents
fields (in other words, those descriptors with events or errors reported).
A value of 0 indicates that the call timed out and no file
descriptors were ready.
On error, -1 is returned, and
errno
is set appropriately.

ERRORS

EFAULT

The array given as argument was not contained in the calling program's
address space.

VERSIONS

The
poll()
system call was introduced in Linux 2.1.23.
The
poll()
library call was introduced in libc 5.4.28
(and provides emulation using select(2) if your kernel does not
have a
poll()
system call).

The
ppoll()
system call was added to Linux in kernel 2.6.16.
The
ppoll()
library call was added in glibc 2.4.

CONFORMING TO

poll()
conforms to POSIX.1-2001.
ppoll()
is Linux-specific.

NOTES

Some implementations define the nonstandard constant
INFTIM
with the value -1 for use as a
timeout
for
poll().
This constant is not provided in glibc.

Linux Notes

The Linux
ppoll()
system call modifies its
timeout_ts
argument.
However, the glibc wrapper function hides this behavior
by using a local variable for the timeout argument that
is passed to the system call.
Thus, the glibc
ppoll()
function does not modify its
timeout_ts
argument.

BUGS

See the discussion of spurious readiness notifications under the
BUGS section of
select(2).